RF switch circuit including a series connection of a plurality of transistors, RF switch including an RF switch circuit and method for switching RF signals

ABSTRACT

An RF switch circuit for switching RF signals includes a first terminal and a second terminal and a series connection of a plurality of transistors between the first terminal of the RF switch circuit and the second terminal of the RF switch circuit. Furthermore, the RF switch circuit includes a control circuit configured to conductively couple, in a high impedance state of the RF switch circuit, the first terminal of the RF switch circuit to a control terminal of a first transistor in a series of the series connection of the plurality of transistors. The second terminal of the RF switch circuit is conductively coupled to a control terminal of a last transistor in the series of the series connection of the plurality of transistors.

This is a continuation application of U.S patent application Ser. No.13/247,233, which was filed on Sep. 28,2011.which has issued as U.S Pat.No. 8,587,361, and which is hereby incorporated herein by reference inits entirety.

TECHNICAL FIELD

Embodiments of the present invention relate to an RF switch circuit.Further embodiments of the present invention relate to an RF switchcomprising such an RF switch circuit, which may be used as a frontendswitch in mobile phones.

BACKGROUND

An RF switch should comprise a small insertion loss, high powerstability and a good linearity. A conventional switch can be built basedon silicon. For this a charge pump on the chip is required, becausegates in deactivated branches have to be biased with a negative voltageto ensure voltage stability. Furthermore, the substrate has to be biasedwith a negative voltage when compared to the source drain nodes of thetransistors, such that the linearity of the transistors is ensured. Thedisadvantage is the integration of the charge pump, which consumes areaand additional process steps, which results in a more expensive chip.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide an RF switch circuit forswitching RF signals. The RF switch circuit comprises a first terminaland a second terminal. Furthermore, the RF switch circuit comprises aseries connection of a plurality of transistors between the firstterminal of the RF switch circuit and the second terminal of the RFswitch circuit.

Furthermore, the RF switch circuit comprises a control circuitconfigured to conductively couple, in a high impedance state of the RFswitch circuit, the first terminal of the RF switch circuit to a controlterminal of a first transistor in a series of the series connection ofthe plurality of transistors. Furthermore, the control circuit isconfigured to conductively couple, in the high impedance state of the RFswitch circuit, the second terminal of the RF switch circuit to acontrol terminal of a last transistor in the series of the seriesconnection of the plurality of transistors.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be described in the followingusing the accompanying figures, in which:

FIG. 1 shows a schematic illustration of an RF switch circuit accordingto an embodiment of the present invention;

FIG. 2 a shows a schematic illustration of an RF switch circuitaccording to a further embodiment of the present invention in anoff-state;

FIG. 2 b shows a more detailed illustration of the voltages which canoccur at a last transistor in a series of a series connection of the RFswitch circuit from FIG. 2 a in the off-state;

FIG. 2 c shows the RF switch circuit from FIG. 2 a in the on-state;

FIG. 3 a shows a schematic illustration of an RF switch according to afurther embodiment of the present invention;

FIG. 3 b shows a modification of the RF switch from FIG. 3 a accordingto a further embodiment of the present invention; and

FIG. 4 shows a flow diagram of a method according to a furtherembodiment of the present invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Before embodiments of the present invention will be described in detailusing the accompanying figures it is to be pointed out that the same orfunctionally equal elements are provided with the same referencenumbers. Hence, descriptions provided for elements having the samereference numbers are mutually exchangeable.

In the present application the meaning of coupling is in the sense of adirect low impedance coupling and an indirect coupling with one or moreelements in-between, such that a signal at a second node is dependent ona signal at a first node which is coupled to the second node. In otherwords, further elements, especially switching elements (liketransistors) or drivers, may be placed between two coupled elements.Between two coupled elements an additional element may be placed, butnot necessarily need to, therefore two coupled elements may be directlyconnected (using a low impedance connection, like a wire or a trace or aconductor path).

FIG. 1 shows a schematic illustration of an RF switch circuit 100 forswitching RF signals 101 according to an embodiment of the presentinvention. The RF switch circuit 100 comprises a first terminal 103 anda second terminal 105. Furthermore, the RF switch circuit 100 comprisesa series connection 107 of a plurality of transistors between the firstterminal 103 of the RF switch circuit 100 and the second terminal 105 ofthe RF circuit 100.

Furthermore, the RF switch circuit 100 comprises a control circuit 109configured to conductively couple, in a high impedance state (or anon-conductive state) of the RF switch circuit 100, the first terminal103 of the RF switch circuit 100 to a control terminal 111 a of a firsttransistor 111 in a series of the series connection 107 of the pluralityof transistors. Furthermore, the control circuit 109 is configured tocouple, in the high impedance state of the RF switch circuit 100, thesecond terminal 105 of the RF switch circuit 100 to a control terminal113 a of a last transistor 113 in the series of the series connection107 of the plurality of transistors.

The first transistor 111 of the series connection 107 comprises thecontrol terminal 111 a, a first terminal 111 b and a second terminal 111c. The last transistor 113 of the series connection 107 comprises thecontrol terminal 113 a, a first terminal 113 b and a second terminal 113c. A channel of the first transistor 111 of the series connection 107extends between the first terminal 111 b and the second terminal 111 cof the first transistor 111 of the series connection 107 and a channelof the last transistor 113 of the series connection 107 extends betweenthe first terminal 113 b and the second terminal 113 c of the lasttransistor 113 of the series connection 107. The control circuit 109 maybe configured to bring the channels of the first transistor 111 and thelast transistor 113 of the series connection 107 into a high impedancestate (or a non-conductive state) by applying a first potential (e.g.,ground potential for n channel transistors) and to bring the channels ofthe first transistor 111 and the last transistor 113 of the seriesconnection 107 into a low impedance state (or conductive state) byapplying a second potential (e.g., supply potential for n channeltransistors) to the control terminals 111 a, 113 a of the firsttransistor 111 and the last transistor 113 of the series connection 107.During the high impedance state of the channels of the first transistor111 and the last transistor 113 of the series connection 107 the RFswitch circuit is in its high impedance state, i.e., no RF signals 101can be routed from the first terminal 103 to the second terminal 105 ofthe RF switch circuit 100 or in the other direction.

In the low impedance state of the channels of the first transistor 111and the last transistor 113 of the series connection 107 the RF switchcircuit 100 is in its low impedance state, i.e., RF signals 101 may berouted from the first terminal 103 to the second terminal 105 or in theother direction from the second terminal 105 to the first terminal 103of the RF switch circuit 100.

The first terminal 103 of the RF switch circuit 100 may be coupled tothe first terminal 111 b of the first transistor 111 of the seriesconnection 107 or may be formed by the first terminal 111 b of the firsttransistor 111 of the series connection 107, such that, in the highimpedance state of the RF switch circuit 100 (when the control terminal111 a of the first transistor 111 of the series connection 107 isconductively coupled to the first terminal 103 of the RF switch circuit100), the control terminal 111 a and the first terminal 111 b of theseries connection 107 are shortened.

Hence, a parasitic capacitance 111 d between the control terminal 111 aand the first terminal 111 b of the first transistor 111 of the seriesconnection 107 is bypassed. This parasitic capacitance 111 d may be agate source capacitance of the first transistor 111 of the seriesconnection 107.

Furthermore, the second terminal 105 of the RF switch circuit 100 may becoupled to the second terminal 113 c of the last transistor 113 of theseries connection 107 or may be formed by the second terminal 113 c ofthe last transistor 113 of the series connection 107, such that, in thehigh impedance state of the RF switch circuit 100 (during which thecontrol terminal 113 a of the last transistor 113 of the seriesconnection 107 is conductively coupled to the second terminal 105 of theRF switch circuit 100), a parasitic capacitance 113 e (e.g., a gatedrain capacitance) between the control terminal 113 a and the secondterminal 113 c of the last transistor 113 of the series connection 107is bypassed.

This bypassing of the parasitic capacitances 111 d, 113 e at theterminals 103, 105, which may be an input terminal and an outputterminal of the RF switch circuit 100 enables a biasing of innerterminals 111 c, 113 b of the plurality of transistors of the seriesconnection 107 with a positive potential (e.g., supply potential). Inthe example shown in FIG. 1 the inner terminals of the plurality oftransistors of the series connection 107 are the second terminal 111 cof the first transistor 111 and the first terminal 113 b of the lasttransistor 113 of the series connection 107. The first terminal 111 b ofthe first transistor 111 and the second terminal 113 c of the lasttransistor 113 of the series connection 107 form outer terminals of theseries connection 107.

By biasing the inner terminals 111 c, 113 b of the plurality oftransistors of the series connection 107, a negative voltage of thesubstrate when compared to the inner terminals 111 c, 113 b (which maybe source drain nodes or source drain areas) can be achieved without theneed for a negative substrate biasing of the substrate of the pluralityof transistors of the series connection 107.

Hence, an area consuming and current consuming charge pump forgenerating a negative bias potential can be omitted in the RF switchcircuit 100.

By conductively coupling, during the high impedance state of the RFswitch circuit 100, the control terminal 111 a of the first transistor111 of the series connection 107 to the first terminal 103 of the RFswitch circuit 100, it is achieved that, even with the positive biasingof the inner terminals 111 c, 113 b of the plurality of transistors ofthe series connection 107, the channel of the first transistor 111 ofthe series connection 107 stays in its high impedance state for thecomplete range of the RF signals 101. The same applies for the lasttransistor 113 of the series connection 107.

Without the principle shown in FIG. 1 of shortening the controlterminals 111 a, 113 a of the outer transistors 111, 113 of the seriesconnection 107 with the terminals 103, 105 of the RF switch circuit 100,it could happen that for certain voltages of the RF signal 101, channelsof the outer transistors 111, 113 would switch between the highimpedance state and the low impedance state with the frequency of the RFsignals 101. Hence, harmonics would be generated which would decreasethe performance of the RF switch circuit. But as described above, thisis prevented by conductively coupling or connecting the controlterminals 111 a, 113 a (or gates 111 a, 113 a) of the outer transistors111, 113 of the series connection 107 with the terminals 103, 105 of theRF switch circuit 100.

Hence, no harmonics are generated during the high impedance state of theRF switch circuit 100. Furthermore, a high linearity of the RF switchcircuit 100 is achieved by biasing the inner terminals 111 c, 113 b ofthe plurality of transistors of the series connection 107 with a higherpotential than a potential of the substrate of the plurality oftransistors of the series connection 107.

Hence, the substrate of the plurality of transistors of the seriesconnection 107 can be conductively coupled, in the high impedance stateof the RF switch circuit 100 and in the low impedance state of the RFswitch circuit 100, to a ground terminal VSS of the RF switch circuit100, at which a ground potential is provided. In other words, thesubstrate may permanently have ground potential.

The control circuit 109 may be configured to conductively couple thecontrol terminal 111 a of the first transistor 111 of the seriesconnection 107 to the first terminal 103 of the RF switch circuit 100 bymeans of a first switch or switching element 115. The control circuit109 may bring the first switch 115 into a low impedance state (orconductive state) during the high impedance state of the RF switchcircuit 100 for conductively coupling the first terminal 103 of the RFswitch circuit 100 and the control terminal 111 a of the firsttransistor 111 of the series connection 107. Furthermore, in the lowimpedance state of the RF switch circuit 100, the control circuit 109may conductively decouple the control terminal 111 a of the firsttransistor 111 of the series connection 107 and the first terminal 103of the RF switch 100, e.g., by bringing the first switch 115 into a highimpedance state (or a non conductive state).

Furthermore, the control circuit 109 may be configured to conductivelycouple the control terminal 113 a of the last transistor 113 of theseries connection 107 to the second terminal 105 of the RF switchcircuit 100 by means of a second switch 117 or switching element 117.Hence, during the high impedance state of the RF switch circuit 100, thecontrol circuit 109 may bring the second switch 117 into a low impedancestate (or conductive state), for conductively coupling the controlterminal 113 a of the last transistor 113 of the series connection 107to the second terminal 105 of the RF switch 100. Furthermore, thecontrol circuit 109 may be configured to conductively decouple, in thelow impedance state of the RF switch circuit 100, the control terminal113 a of the last transistor 113 of the series connection 107 and thesecond terminal 105 of the RF switch circuit 100, e.g., by bringing thesecond switch 117 into a high impedance state (or a non conductivestate).

According to further embodiments, the control circuit 109 may beconfigured to conductively couple, in the high impedance state of the RFswitch circuit 100, the second terminal 105 of the RF switch circuit 100to the ground terminal VSS of the RF switch circuit 100 at which theground potential is provided. As an example, the RF switch circuit 100may comprise a shunt switch 119 coupled between the ground terminal VSSand the second terminal 105 of the RF switch circuit 100, which iscontrolled by the control circuit 109, such that the control circuit 109brings the shunt switch 119 into a low impedance state (or conductivestate) during the high impedance state of the RF switch circuit 100 andinto a high impedance state (or a non conductive state) during the lowimpedance state of the RF switch circuit 100. As an example, the shuntswitch 119 may be realized by a further series connection of a furtherplurality of transistors coupled between the second terminal 105 of theRF switch 100 and the ground terminal VSS.

The control circuit 109 may be configured to bring, in the highimpedance state of the RF switch circuit 100, channels of this furtherplurality of transistors into a conductive state and to bring, in thelow impedance state of the RF switch circuit 100, the channels of thefurther plurality of transistors into a non conductive state.

According to further embodiments, the control circuit 109 may beconfigured to bring, in the low impedance state of the RF switch circuit100 (when RF signals can be routed from the first terminal 103 to thesecond terminal 105 of the RF switch circuit 100 or vice versa), thesecond terminal 111 c of the first transistor 111 of the seriesconnection 107 and the first terminal 113 b of the last transistor 113of the series connection 107 into an unbiased state. In other words, inthe low impedance state of the RF switch circuit 100, the controlcircuit 109 may remove the biasing of the inner terminals 111 c, 113 bof the plurality of transistors of the series connection 107.

In the following, a more detailed example of embodiments of the presentinvention is given using the FIGS. 2 a to 2 c.

The voltages and potentials named in the following always refer to theground potential having 0 volts. In the following a supply potential of+3 volts is assumed and a ground potential of 0 volts is assumed.Furthermore, it is assumed that the RF signals 101 have a maximumamplitude of +3 volts and a minimum amplitude of −3 volts.

FIG. 2 a shows a schematic illustration of an RF switch circuit 200according to a further embodiment of the present invention, the RFswitch circuit 200 extends the RF switch circuit 100 in that a seriesconnection 107′ of the RF switch 200 comprises three additional innertransistors 201, 203, 205 which are in a series of the series connection107′ arranged between the first transistor 111 and the last transistor113 of the series connection 107′.

In general, the number of inner transistors may be chosen arbitrarilyand may be equal to or larger than 0.

The control circuit 109 may be coupled to control terminals 111 a, 201a, 203 a, 205 a, 113 a of the plurality of transistors 111, 201, 203,205, 113 of the series connection 107′ by means of resistors R3.

As can be seen from FIG. 2 a inner terminals 111 c, 207, 209, 113 b orsource drain areas of the plurality of transistors 111, 201, 203, 205,113 of the series connection 107′ are shared by two transistors. Hence,each of the inner terminals 111 c, 207, 209, 113 b of the seriesconnection 107′ forms a source region for a transistor and a drainregion for another transistor.

The control circuit 109 may be coupled to the inner terminals or innersource drain nodes 111 c, 207, 209, 113 b by means of resistors R4.

As an example, the inner terminal 111 c may form a drain region of thefirst transistor 111 and a source region of a second transistor 201being arranged in the series of the series connection 107′ subsequent tothe first transistor 111 of the series connection 107′. Furthermore, theinner terminal 113 b may form a drain region of a third transistor 205being arranged in the series of the series connection 107′ directlybefore the last transistor 113 and may form a source region of the lasttransistor 113 of the series connection 107′.

As can be seen, the inner terminals 111 c, 207, 209, 113 b or innersource drain regions of the plurality of transistors 111, 201, 203, 205,113 of the series connection 107′ can be biased by the control circuit109.

In the example shown in FIG. 2 a, the RF switch circuit 200 is shown inits off-state or high impedance state. Hence, in this high impedancestate, as described before, the control circuit 109 may be configured tobias the inner terminals 111 c, 113 b, 207, 209 of the plurality oftransistors of the series connection 107′ with a positive supplypotential (in the case shown in FIG. 2 a 3 volts).

Furthermore, in the example shown in FIG. 2 a, the first switch 115 isformed by a first switching transistor 115, a first terminal 115 b ofwhich is connected to the first terminal 103 of the RF circuit 200 and asecond terminal 115 c of which is connected to the control gate 111 a ofthe first transistor 111 of the series connection 107′. A controlterminal 115 a of the first switching transistor 115 is coupled, e.g.,by means of a first resistor R1 to the control circuit 109.

Furthermore, the second switch 117 is formed by a second switchingtransistor 117, a first terminal 117 b of which is connected to thecontrol terminal 113 a of the last transistor 113 of the seriesconnection 107′ and a second terminal 117 c of which is connected to thesecond terminal 105 of the RF switch circuit 200. A control terminal 117a of the second switching transistor 117 is coupled, e.g., by means of asecond resistor R2 to the control circuit 109.

The control circuit 109 may be configured to conductively couple, in thehigh impedance state of the RF switch circuit 200, as shown in FIG. 2 a,the control terminal 111 a of the first transistor 111 of the seriesconnection 107′ and the first terminal 103 of the RF switch circuit 200by applying a suitable potential to the control terminal 115 a of thefirst switching transistor 115, such that the first switching transistor115 is brought into its low impedance state.

Furthermore, the control circuit 109 may be configured to conductivelycouple, in the high impedance state of the RF switch circuit 200, thecontrol terminal 113 a of the last transistor 113 of the seriesconnection 107′ and the second terminal 105 of the RF switch circuit 200by applying a suitable voltage to the control terminal 117 a of thesecond switching transistor 117, such that the second switchingtransistor 117 is brought into its low impedance state.

In the example shown in FIG. 2 a, the switching transistors 115, 117 aren channel transistors. Hence, the control circuit 109 is configured toapply the positive supply potential to the switching transistor 115, 117for bringing them into their low impedance states.

Furthermore, the plurality of transistors of the series connection 107′may be n channel transistors. Hence, the control circuit 109 may beconfigured to apply ground potential to the control terminals 111 a, 201a, 203 a, 205 a, 113 a of the plurality of transistors 111, 201, 203,205, 113 to bring their channels into their high impedance states.

FIG. 2 b shows the last transistor 113 of the series connection 107′during the off-state condition of the RF switch circuit 200. Hence, thesecond switching transistor 117 is in a low impedance state andconductively couples the control terminal 113 a of the last transistor113 of the series connection 107′ and the second terminal 113 c of thelast transistor 113 of the series connection 107′ (and the secondterminal 105 of the RF switch circuit 200). Hence, the gate draincapacitance 113 e of the last transistor 113 of the series connection107′ between the control terminal 113 a and the second terminal 113 c ofthe last transistor 113 is bypassed.

In a further embodiment of the present invention, the second terminal113 c of the last transistor 113 of the series connection 107′ may bepulled to ground potential by means of the shunt switch 119 or the shuntpath 119, during the high impedance state of the RF switch circuit 200,the gate terminal 113 a of the last transistor 113 of the seriesconnection 107′ has also ground potential independent of a potential atthe first terminal 113 b of the last transistor 113 of the seriesconnection 107′.

As can be seen from FIG. 2 b a source gate capacitance 113 d of the lasttransistor 113 of the series connection 107′ is not bypassed.Furthermore, the first terminal 113 b of the last transistor 113 of theseries connection 107′ is biased with the supply potential (in this case+3 volts).

A maximum amplitude of the RF signal at the first terminal 113 b of thelast transistor 113 of the series connection 107′ may be +3 volts and aminimum amplitude may be −3 volts.

To limit the voltages of the RF signal to these values, the RF switchcircuit 200 may comprise a voltage limiter, for example, at the firstterminal 103 of the RF switch circuit 200.

It is to be pointed out that by having the series connection 107′ of theplurality of transistors 111, 201, 203, 205, 113 the RF voltage or thevoltage of an RF signal, e.g., at the first terminal 103 or the secondterminal 105 is divided over the plurality of transistors 111, 201, 203,205, 113, such that at each transistor 111, 201, 203, 205, 113 only a(small) RF-voltage occurs which does not exceed the threshold voltage ofthe transistors 111, 201, 203, 205, 113 (so called stacking oftransistors). This series connection 107′ of the plurality oftransistors 111, 201, 203, 205, 113 in the off-state can be describedwith a series connection of capacitances, in which each capacitancerepresents a Drain-Gate-capacitance or a Gate-Source-capacitance of atransistor of the plurality of transistors 111, 201, 203, 205, 113. Thehigh frequency (signal) is divided approximately linear, such that ateach transistor 111, 201, 203, 205, 113 only a small voltage dropoccurs. The more transistors are connected (or stacked) in series, thesmaller the voltage at each transistor.

In the example shown in FIG. 2 b the positive half wave of the RF signalresults in a maximum gate source voltage between the gate terminal 113 aand the first terminal 113 b of the last transistor 113 of:0 volts−6 volts=−6 volts.

The negative half wave of the RF signal results in a maximum gate sourcevoltage of:0 volts−0 volts=0 volts.

Furthermore, the positive half wave of the RF signal results in amaximum gate drain voltage between the control terminal 113 a and thesecond terminal 113 c of the last transistor 113 of:0 volts−0 volts=0 volts.

The negative half wave of the RF signal results in a maximum gate drainvoltage of:0 volts−0 volts=−0 volts.

As can be seen, even with a small threshold voltage, e.g. VTH=0.5 volts,the last transistor 113 remains in its non conductive state for thecomplete range of the incoming RF signal.

As an example, in the following it is described what would happen if thegate drain capacitance 113 e would not be bypassed by the secondswitching transistor 117.

In this case, the positive half wave of the incoming RF signal wouldresult in a maximum gate source voltage of:1.5 volts−6 volts=−4.5 volts.

The negative half wave of the RF signal would result in a maximum gatesource voltage of:−1.5 volts−0 volts=−1.5 volts.

Furthermore, the positive half wave of the incoming RF signal wouldresult in a maximum gate drain voltage of:1.5 volts−0 volts=1.5 volts.

In this case, due to the higher potential at the gate terminal 113 awhen compared to the second terminal 113 c the transistor 113 wouldopen, i.e., the channel of the last transistor 113 would becomeconductive. This would generate harmonics in the RF switch circuit.

As described before, this problem is solved by bypassing the gate draincapacitance 113 e of the last transistor 113 by means of the secondswitching transistor 117.

The same principle as described in FIG. 2 b applies vice versa for thefirst transistor 111 in conjunction with the first switching transistor115.

FIG. 2 c shows the RF switch circuit 200 from FIG. 2 a in the lowimpedance state or on-state. In this low impedance state the controlcircuit 109 applies suitable voltages to the control terminal 115 a ofthe first switching transistor 115 and to the control terminal 117 a ofthe second switching transistor 117, such that the first switchingtransistor 115 and the second switching transistor 117 are in the highimpedance states. In the example shown in FIG. 2 c the control circuit109 applies ground potential to the control terminals 115 a, 117 a.

Furthermore, the control circuit 109 applies suitable voltages to thecontrol terminals 111 a, 201 a, 203 a, 205 a, 113 a of the plurality oftransistors of the series connection 107′ for bringing the plurality oftransistors (or their channels) in the conductive state, such that RFsignals 101 can be routed from the first terminal 103 of the RF switchcircuit 200 to the second terminal 105 of the RF switch circuit 200 orvice versa. In the example shown in FIG. 2 c, the control circuit 109applies supply potential to the control terminals 111 a, 201 a, 203 a,205 a, 113 a.

Furthermore, in the low impedance state of the RF switch circuit 200,the control circuit 109 keeps the inner terminals 111 c, 207, 209, 113 bin an unbiased state.

As can be seen from FIGS. 2 a and 2 c, a substrate terminal of thesubstrate of the plurality of transistors is not biased, i.e., hasground potential in the high impedance state of the RF switch circuit200 and in the low impedance state of the RF switch circuit 200. Hence,no charge pump is needed for producing a negative biasing potential forthe substrate.

To summarize, in the RF switch circuit 200, the inner source drain areasor terminals 111 c, 207, 209, 113 b are biased with a positive voltage(during the high impedance state of the RF switch circuit 200). Thisleads to a bias when compared to the substrate of the plurality oftransistors. The outer terminals or outer nodes, 111 b, 113 c of theouter transistors 111, 113 of the series connection 107′ still have 0volts. To prevent that the outer transistors 111, 113 with a lowthreshold voltage Vth (e.g., of 0.5 volts) switch on in response to acertain amplitude of the RF signals 101, which would generate harmonics,the gate terminals 111 a, 113 a of the outer transistors 111, 113 areconnected to their outer source drain nodes 111 b, 113 c with theswitching transistors 115, 117.

In the on state of the RF switch circuit 200, no voltage exists betweenthe inner source drain nodes 111 c, 207, 209, 113 b and the substrate.

To summarize, in the case that the plurality of transistors of theseries connection 107′ are n channel transistors, the control circuit109 may be configured to apply, in the high impedance state of the RFswitch circuit 200, the ground potential to the control terminal 111 aof the first transistor 111 and to the control terminal 113 a of thelast transistor 113. Furthermore the control circuit 109 is configuredto apply the positive supply potential (or a positive bias potential) tothe inner terminals 111 c, 207, 209, 113 b. The positive bias potentialcan be chosen such that a difference between a maximum RF signalpotential (in the example in FIG. 2 a e.g. +3 volts) at the firstterminal 111 b of the first transistor 111 and the positive biaspotential is smaller than the threshold voltage of the first transistor111.

Furthermore, the positive bias potential may be chosen such that adifference between the ground potential and a sum of the bias potentialand the minimum RF signal potential (in the example shown in FIG. 2 a −3volts) at the first terminal 113 b of the last transistor 113 is smallerthan the threshold voltage of the last transistor 113. By choosing thepositive bias potential as described above, it can be achieved, that forthe complete range of the RF signals 101, the first transistor 111 andthe last transistor 113 stay, during the high impedance state of the RFswitch circuit 200, in their high impedance states.

As mentioned before, the maximum voltage and the minimum voltage of theRF signals 101 may be limited by a voltage limiter of the RF switchcircuit 200, for example coupled to the first terminal 103 of the RFswitch circuit 200 or to the second terminal 105 of the RF switchcircuit 200 or may be limited by design or an application of the RFswitch circuit 200.

In an application of the RF switch circuit 200, the RF switch circuit200 may be switched between an antenna for receiving and/or transmittingRF signals 101 and a receive or transmit branch of a receiver ortransmitter. As an example, the antenna may be coupled to the firstterminal 103 of the RF switch circuit and the transmit or receive branchof the transmitter or receiver may be coupled to the second terminal 105of the RF switch circuit.

In some applications an RF switch may be used for switching between atransmit part and a receive part, for this the RF switch may comprisetwo RF switch circuits. FIG. 3 a shows such an RF switch 300 accordingto an embodiment of the present invention. The RF switch 300 comprises afirst RF switch circuit 301 and a second RF switch circuit 302. A firstterminal 103 of the first RF switch circuit 301 and a first terminal 103of the second RF switch circuit 302 are coupled to a common terminal(e.g. antenna terminal) HFP2 of the RF switch 300. A second terminal 105of the first RF switch circuit 301 is coupled to a firsttransmitter/receiver terminal HFP3 of the RF switch 300. A secondterminal 105 of the second RF switch circuit 302 is coupled to a secondtransmitter/receiver terminal HFP1 of the RF switch 300.

In the example shown in FIG. 3 a, the first RF switch circuit 301 andthe second RF switch circuit 302 are identical with the only differencethat control signals SW, SW_for the transistors and switches of thefirst RF switch circuit 301 are inverted to the control signals SW_, SWof the corresponding switches and transistors in the second RF switchcircuit 302. Hence, when the first RF switch circuit 301 is in its highimpedance state, the second RF switch circuit 302 is in its lowimpedance state and when the second RF switch circuit 302 is in its highimpedance state, the first RF switch circuit 301 is in its low impedancestate. The RF switch circuit 301, 302 differ from the RF switch circuitshown in FIG. 1 in that the number of transistors in the seriesconnection is three instead of two as shown in FIG. 1 and in that theshunt switches 119 of the RF switch circuit 301, 302 are eachimplemented with a further series connection of a further plurality oftransistors coupled between the ground terminal VSS and the secondterminals 105 of the RF switch circuits 301, 302. Furthermore, the RFswitch 300 comprises a common control circuit 109 for controlling thefirst RF switch circuit 301 and the second RF switch circuit 302.

Furthermore, the control circuit 109 provides the biasing potentials forthe inner nodes of the plurality of transistors of the series connectionof the first RF switch circuit 301 and the second RF switch circuit 302.As can be seen from FIG. 3 a, the biasing for the first RF switchcircuit 301 is inverted to the biasing of the second RF switch circuit302. Hence, the inner nodes of the plurality of transistors of theseries connection of the first RF switch circuit 301 are biased when theinner nodes of the plurality of transistors of the series connection ofthe second RF switch circuit 302 are in an unbiased state and viceversa.

FIG. 3 b shows a modification of the RF switch 300 from FIG. 3 aaccording to a further embodiment of the present invention. In thisembodiment, the inner source drain nodes of the transistors of the shuntpaths 119 are pulled to ground potential VSS by means of (highimpedance) resistors R5.

The transistors described above may be, for example, field effecttransistors (FET), metal oxide semiconductor field effect transistors(MOSFETs), bipolar transistors, high electron mobility transistors(HEMTs), hetero bipolar transistors (HBTs), metal insulator field effecttransistors (MISFETs), silicon-on-insulator transistors (SOI) orsilicon-on-sapphire transistors (SOS).

A first terminal of a transistor can be, for example, a source terminalor a drain terminal or an emitter terminal or a collector terminal ofthe transistor. A second terminal can be, for example, a drain terminalor a source terminal or a collector terminal or an emitter terminal ofthe transistor. A control terminal can be, for example, a gate terminalor a base terminal of the transistor. A channel of the transistor may bea drain source path of the transistor or an emitter collector path ofthe transistor. Hence, typically, a main transistor current is routedfrom the first terminal to the second terminal or vice versa.

Although in the examples described above, the transistors are n channeltransistors, complementary realizations are possible.

Embodiments of the present invention can be used, for example, in RFswitches, or HIPAC-ICs (HIPAC-Highly integrated passive and activecomponents).

FIG. 4 shows a flow diagram of a method 400 according to an embodimentof the present invention. The method 400 for switching RF signals may beperformed using an RF switch circuit according to an embodiment of thepresent invention (e.g., the RF switch circuit 100). In other words, themethod 400 may be used for switching RF signals by means of an RF switchcircuit comprising a first terminal and a second terminal and a seriesconnection of a plurality of transistors between the first terminal ofthe RF switch circuit and the second terminal of the RF switch circuit.

The method 400 comprises a step 401 of conductively coupling, in a highimpedance state of the RF switch circuit, the first terminal of the RFswitch circuit to a control terminal of a first transistor in the seriesof the series connection of the plurality of transistors and the secondterminal of the RF switch circuit to a control terminal of a lasttransistor in the series of the series connection of the plurality oftransistors.

The method 400 may be supplemented by any of the features andfunctionalities described herein with respect to the apparatus, and maybe implemented using the hardware components of the apparatus.

Although some aspects have been described in the context of anapparatus, it is clear that these aspects also represent a descriptionof the corresponding method, where a block or device corresponds to amethod step or a feature of a method step. Analogously, aspectsdescribed in the context of a method step also represent a descriptionof a corresponding block or item or feature of a correspondingapparatus. Some or all of the method steps may be executed by (or using)a hardware apparatus, like for example, a microprocessor, a programmablecomputer or an electronic circuit. In some embodiments, one or more ofthe most important method steps may be executed by such an apparatus.

The inventive encoded audio signal can be stored on a digital storagemedium or can be transmitted on a transmission medium such as a wirelesstransmission medium or a wired transmission medium such as the Internet.

Depending on certain implementation requirements, embodiments of theinvention can be implemented in hardware or in software. Theimplementation can be performed using a digital storage medium, forexample, a floppy disk, a DVD, a Blue-Ray, a CD, a ROM, a PROM, anEPROM, an EEPROM or a FLASH memory, having electronically readablecontrol signals stored thereon, which cooperate (or are capable ofcooperating) with a programmable computer system such that therespective method is performed. Therefore, the digital storage mediummay be computer readable.

Some embodiments according to the invention comprise a data carrierhaving electronically readable control signals, which are capable ofcooperating with a programmable computer system, such that one of themethods described herein is performed.

Generally, embodiments of the present invention can be implemented as acomputer program product with a program code, the program code beingoperative for performing one of the methods when the computer programproduct runs on a computer. The program code may, for example, be storedon a machine readable carrier.

Other embodiments comprise the computer program for performing one ofthe methods described herein, stored on a machine readable carrier.

In other words, an embodiment of the inventive method is, therefore, acomputer program having a program code for performing one of the methodsdescribed herein, when the computer program runs on a computer.

A further embodiment of the inventive methods is, therefore, a datacarrier (or a digital storage medium, or a computer-readable medium)comprising, recorded thereon, the computer program for performing one ofthe methods described herein. The data carrier, the digital storagemedium or the recorded medium are typically tangible and/ornon-transitionary.

A further embodiment of the inventive method is, therefore, a datastream or a sequence of signals representing the computer program forperforming one of the methods described herein. The data stream or thesequence of signals may, for example, be configured to be transferredvia a data communication connection, for example, via the Internet.

A further embodiment comprises a processing means, for example, acomputer, or a programmable logic device, configured to or adapted toperform one of the methods described herein.

A further embodiment comprises a computer having installed thereon thecomputer program for performing one of the methods described herein.

A further embodiment according to the invention comprises an apparatusor a system configured to transfer (for example, electronically oroptically) a computer program for performing one of the methodsdescribed herein to a receiver. The receiver may, for example, be acomputer, a mobile device, a memory device or the like. The apparatus orsystem may, for example, comprise a file server for transferring thecomputer program to the receiver.

In some embodiments, a programmable logic device (for example, a fieldprogrammable gate array) may be used to perform some or all of thefunctionalities of the methods described herein. In some embodiments, afield programmable gate array may cooperate with a microprocessor inorder to perform one of the methods described herein. Generally, themethods are preferably performed by any hardware apparatus.

The above described embodiments are merely illustrative for theprinciples of the present invention. It is understood that modificationsand variations of the arrangements and the details described herein willbe apparent to others skilled in the art. It is the intent, therefore,to be limited only by the scope of the impending patent claims and notby the specific details presented by way of description and explanationof the embodiments herein.

Although each claim only refers back to one single claim, the disclosurealso covers any conceivable combination of claims.

What is claimed is:
 1. An RF switch circuit for switching RF signals, the RF switch circuit comprising: a first terminal and a second terminal; a series connection of a plurality of transistors between the first terminal of the RF switch circuit and the second terminal of the RF switch circuit, wherein a first terminal of the RF switch circuit is connected to a first terminal of a first transistor in a series of the series connection of the plurality of transistors and a second terminal of the RF switch circuit is connected to a second terminal of a last transistor in the series of the series connection of the plurality of transistors; a first switch directly connected between the first terminal of the first transistor and a control terminal of the first transistor; a second switch directly connected between the second terminal of the second transistor and a control terminal of the second transistor; and a control circuit configured to close the first switch and the second switch when the RF switch circuit is an a high impedance state, and open the first switch and the second switch when the RF switch circuit is in a low impedance state.
 2. The RF switch circuit according to claim 1, wherein the control circuit is configured to bias, in the high impedance state of the RF switch circuit, a second terminal of the first transistor and a first terminal of the last transistor with a positive supply potential.
 3. The RF switch circuit according to claim 2, wherein the control circuit is further configured to bias, in the high impedance state of the RF switch circuit, first terminals and second terminals of transistors of the plurality of transistors arranged in the series connection between the first transistor and the last transistor with the positive supply potential.
 4. The RF switch circuit according to claim 2, wherein the control circuit is configured to bring, in the low impedance state of the RF switch circuit, the second terminal of the first transistor and the first terminal of the last transistor into an unbiased state.
 5. The RF switch circuit according to claim 4, wherein the control circuit is further configured to bring, in the low impedance state of the RF switch circuit, first terminals and second terminals of transistors of the plurality of transistors arranged in the series connection between the first transistor and the last transistor, into an unbiased state.
 6. The RF switch circuit according to claim 1, wherein a substrate terminal of a substrate of the plurality of transistors is, in the high impedance state of the RF switch circuit and the low impedance state of the RF switch circuit, conductively coupled to a ground terminal of the RF switch circuit, at which a ground potential is provided.
 7. The RF switch circuit according to claim 1, wherein the control circuit is configured to apply, in the high impedance state of the RF switch circuit, a positive bias potential to a second terminal of the first transistor; and wherein the positive bias potential is chosen such that a difference between a maximum RF signal potential at a first terminal of the first transistor and the positive bias potential is smaller than a threshold voltage of the first transistor.
 8. The RF switch circuit according to claim 7, wherein the control circuit is further configured to apply, in the high impedance state of the RF switch circuit, a positive bias potential to a first terminal of the last transistor and a further bias potential to the control terminal of the first transistor and to the control terminal of the last transistor; and wherein the positive bias potential is chosen such that a difference between the further bias potential and a sum of the positive bias potential and a minimum RF signal potential at the first terminal of the last transistor is smaller than a threshold voltage of the last transistor.
 9. The RF switch circuit according to claim 8, wherein the further bias potential is ground potential.
 10. The RF switch circuit according claim 1, wherein the control circuit is configured to apply, in the high impedance state of the RF switch circuit, a ground potential to a second terminal of the last transistor.
 11. The RF switch circuit according to claim 1, further comprising a further series connection of a further plurality of transistors connected between a second terminal of the last transistor and a ground potential terminal of the RF switch circuit, at which a ground potential is provided.
 12. The RF switch circuit according to claim 11, wherein the control circuit is configured to bring, in the high impedance state of the RF switch circuit, channels of the further plurality of transistors into a conductive state and to bring, in the low impedance state of the RF switch circuit, the channels of the further plurality of transistors into a non-conductive state.
 13. The RF switch circuit according to claim 1, wherein the control circuit is configured to keep, in the high impedance state of the RF switch circuit and in a low impedance state of the RF switch circuit, a first terminal of the first transistor and a second terminal of the last transistor in an unbiased state.
 14. The RF switch circuit according to claim 1, wherein the RF switch circuit is configured to route, in a low impedance state of the RF switch circuit, an incoming RF signal from its first terminal to its second terminal or from its second terminal to its first terminal.
 15. The RF switch circuit according to claim 1, wherein: the first switch comprises a first switching transistor; and the second switch comprises a second switching transistor.
 16. An RF switch circuit comprising: a first terminal and a second terminal; a series connection of a plurality of MOSFETs between the first terminal of the RF switch circuit and the second terminal of the RF switch circuit; a first switching MOSFET and a second switching MOSFET; wherein a first terminal of a first MOSFET in a series of the series connection of the plurality of MOSFETs is connected to the first terminal of the RF switch circuit and a second terminal of a last MOSFET in the series of the series connection of the plurality of MOSFETs is connected to the second terminal of the RF switch circuit; wherein a first terminal of the first switching MOSFET is connected to the first terminal of the first MOSFET and a second terminal of the first switching MOSFET is connected to a gate terminal of the first MOSFET; wherein a first terminal of the second switching MOSFET is connected to a gate control terminal of the last MOSFET and a second terminal of the second switching MOSFET is connected to the second terminal of the last MOSFET; a control circuit coupled to a gate terminal of the first switching MOSFET and a gate terminal of the second switching MOSFET; wherein the control circuit is configured to bring, in a high impedance state of the RF switch circuit, a channel of the first switching MOSFET and a channel of the second switching MOSFET into a conductive state; and wherein the control circuit is further configured to bring, in a low impedance state of the RF switch circuit, the channel of the first switching MOSFET and the channel of the second switching MOSFET into a non-conductive state.
 17. A method of operating an RF switch circuit comprising a first terminal and a second terminal and a series connection of a plurality of transistors between the first terminal of the RF switch circuit and the second terminal of the RF switch circuit, wherein a first terminal of the RF switch is connected to a first terminal of a first transistor in a series of the series connection of the plurality of transistors and a second terminal of the RF switch is connected to a second terminal of a last transistor in the series of the series connection of the plurality of transistors, the method comprising: closing a first switch directly connected between the first terminal of the first transistor and a control terminal of the first transistor when the RF switching circuit is in a high impedance state; closing a second switch directly connected between the first terminal of the first transistor and a control terminal of the first transistor when the RF switching circuit is in the high impedance state; opening the first switch when the RF switching circuit is in a low impedance state; and opening the second switch when the RF switching circuit is in a low impedance state.
 18. An RF switch comprising a first RF switch circuit and a second RF switch circuit; wherein each RF switch circuit comprises a first terminal and a second terminal and wherein each RF switch circuit is configured for switching RF signals; wherein a first terminal of the RF switch is coupled to the first terminals of the RF switch circuits; and wherein at least the first RF switch circuit comprises a series connection of a plurality of transistors between its first terminal and its second terminal, wherein a first terminal of the at least one RF switch circuit is connected to a first terminal of a first transistor in a series of the series connection of the plurality of transistors and a second terminal of the at least one RF switch circuit is connected to a second terminal of a last transistor in the series of the series connection of the plurality of transistors, a first switch directly connected between the first terminal of the first transistor and a control terminal of the first transistor, and a second switch directly connected between the second terminal of the second transistor and a control terminal of the second transistor; and a control circuit configured to close the first switch and the second switch when the at least one RF switch circuit is an a high impedance state, and open the first switch and the second switch when the at least one RF switch circuit is in a low impedance state.
 19. A switch for switching RF signals, comprising: a first terminal and a second terminal; a series connection of a plurality of switching means between the first terminal of the switch for switching RF signals and the second terminal of the switch for switching RF signals, wherein the first terminal of the switch for switching RF signals is connected to a first terminal of a first switching means in a series of the series connection of the plurality of switching means and a second terminal of the switch for switching RF signals is connected to a second terminal of a last switching means in the series of the series connection of the plurality of switching means, a first switch directly connected between the first terminal of the first switching means and a control terminal of the first switching means, and a second switch directly connected between the second terminal of the second switching means and a control terminal of the second switching means; and a means for controlling configured to close the first switch and the second switch when the switch for switching RF signals is an a high impedance state, and open the first switch and the second switch when the switch for switching RF signals is in a low impedance state.
 20. An RF switching circuit comprising: a plurality of series connected transistors comprising a first transistor coupled to a first end of the plurality of series connected transistors and a last transistor coupled to a second end of the plurality of series connected transistors; a plurality of gate resistors comprising first ends coupled to corresponding gates of the plurality of series connected transistors; a first switch directly connected between the first end and the gate of the first transistor; a second switch directly connected between the second end and the gate of the last transistor; and a controller configured to: apply a first deactivation voltage to control nodes of the first switch and the second switch while applying a first activation voltage to the a plurality of series connected transistors via second ends of the plurality of gate resistors, and apply a second activation voltage to the control nodes of the first switch and the second switch while applying a second deactivation voltage to the plurality of series connected transistors via the second ends of the plurality of gate resistors.
 21. The RF switching circuit of claim 20, wherein: the first activation voltage comprises a first positive voltage; the second activation voltage comprises a second positive voltage; the first and second deactivation voltages comprise a ground voltage; a substrate of the plurality of series connected transistors is coupled to the ground voltage; and the controller is further configured to: apply the ground voltage to nodes coupled between the plurality of series connected transistors via a plurality of bias resistors while applying the second activation voltage to the plurality of series connected transistors; and apply a third positive voltage to the nodes coupled between the plurality of series connected transistors via the plurality of bias resistors while applying the second deactivation voltage to the plurality of series connected transistors.
 22. The RF switching circuit of claim 21, wherein the first positive voltage, the second positive voltage and the third positive voltage are a same positive voltage. 